Vertebrate bone is a composite material composed of hydroxyapatite, collagen, and a variety of noncollagenous proteins, as well as embedded and adherent cells. Vertebrate bone can be processed into an implantable biomaterial, such as an allograft, for example, by removing the cells, leaving behind the mineral and extracellular matrix. The processed bone biomaterial can have a variety of properties, depending upon the specific processes and treatments applied to it, and may incorporate characteristics of other biomaterials with which it is combined. For example, bone-derived biomaterials may be processed into load-bearing mineralized grafts that support and integrate with the patient's bone or may alternatively be processed into soft, moldable or flowable demineralizcd bone biomaterials that have the ability to induce a cellular healing response.
The use of bone grafts and bone substitute materials in orthopedic medicine is well known. While bone wounds can regenerate without the formation of scar tissue, fractures and other orthopedic injuries take a long time to heal, during which the bone is unable to support physiologic loading. Metal pins, screws, and meshes are frequently required to replace the mechanical functions of injured bone. However, metal is significantly stiffer than bone. Use of metal implants may result in decreased bone density around the implant site due to stress shielding. Furthermore, metal implants are permanent and unable to participate in physiological remodeling.
Following implantation, the host's own bone remodeling capabilities permit some bone grafts and certain bone substitute materials to remodel into endogenous bone that in most cases is indistinguishable from the host's own bone. In general, however, it is a limitation of allograft bone that larger allografts do not completely remodel, and residual allograft bone may persist at the graft site for many years or indefinitely, potentially acting as a stress riser and a possible fracture site. The use of bone grafts is further limited by the availability of tissue with the appropriate shape and size, as well as the desired mechanical strength and degradation rate.
U.S. Pat. No. 6,294,187, the contents of which are incorporated herein by reference, describes methods for preparing composites including allogenic bone for use in load bearing orthopedic applications. It is desirable to increase the strength of bone-reinforced composites by increasing the strength of the matrix material while retaining the resorbable properties of the matrix. Furthermore, there is a need for a novel resorbable polymer capable of synergistically interacting with bone to make a true composite having mechanical characteristics of both bone and polymer. There is also a need to develop resorbable polymers for the production of bone/polymer composites where the polymer itself has osteopromotive or osteopermissive properties and contributes to osteointegration and remodeling of the composite. It is also desirable to develop implants that do not elicit undesirable immune responses from the recipient. There is also a need to provide composite grafts of suitable shape and size that maximize the utility of the graft tissue.